This invention relates to a phase shifter for use in the field of radio communications, etc., and more particularly to a phase shifter which derives two signals of fixed phase difference without changing the amplitudes of the signals over a wide frequency band.
FIG. 6 is a circuit diagram which shows a prior-art phase shifter referred to as a Nagard type for creating a phase difference of 90.degree. by way of example. In the figure, 1 designates an input terminal, and 2 an amplifier for amplifying a signal from the input terminal 1. 3 and 5 designate resistors between which a capacitor 4 is inserted, and the resistors 3, 5 and the capacitor 4 constitute a series circuit connected across both the ends of the amplifier 2. A capacitor 6 is connected across both the ends of the resistor 3, and a first output terminal 7 is provided at the node of the resistor 3 and the capacitors 4, 6. 8 and 10 designate resistors between which a capacitor 9 is inserted, and the resistors 8, 10 and the capacitor 9 constitute a series circuit connected across both the ends of the amplifier 2. A capacitor 11 is connected across both the ends of the resistor 8, and a second output terminal 12 is provided at the node of the resistor 8 and the capacitors 9, 11.
Here, it is assumed that 4R.sub.1 C.sub.1 =R.sub.2 C.sub.2 be satisfied when the gain of the amplifier 2 is set at -(7/2) and when as circuit constants, the resistances of the resistors 5, 10, 3 and 8 are R.sub.1, R.sub.2, (4/3) R.sub.1 and (4/3) R.sub.2 respectively, while the capacitances of the capacitors 6, 11, 4 and 9 are C.sub.1, C.sub.2, 2C.sub.1 and 2C.sub.2 respectively.
At this time, the characteristic of the phase shifter in FIG. 6 is as shown in FIG. 7 wherein the phase is taken on the axis of ordinates and the frequency on the axis of abscissas. As seen from FIG. 7, the frequency band width in which the phase shifter operates effectively as a 90.degree. phase shifter is about one decade (250-2500 Hz), and the phase precision (deviations from 90.degree.) on that occasion is about .+-.1.degree.. Analysis has also revealed that the alteration of the circuit constants can improve the phase precision, but that it narrows the effective band width.
Since, as described above, the prior-art phase shifter has the frequency band width and the phase precision restricted by one set of circuit constants, it has had the problem that more improvements in performance cannot be expected.